Proper error reporting in scriptloader for all browsers.

[kkrzton]

While working on #13943 there was a need to check if config scripts were loaded successfully. It is partially supported by scriptloader but not for all browsers (IE < 11, Safari, Opera - probably before migrating to Webkit). It is not really supported by those browsers in a proper way but still it may be needed in the future so probably should be done.

if ( CKEDITOR.env.ie && CKEDITOR.env.version < 11 ) {
    // FIXME: For IE, we are not able to return false on error (like 404).
    script.$.onreadystatechange = function() {
        ...
} else {
    ...

    // FIXME: Opera and Safari will not fire onerror.
    script.$.onerror = function() {
        ...

part of scriptloader.js loadScript function with planned fixes

[kkrzton]

Changes in ​t/14275.

There is no any nice & clean solution for IE8 so it works as before in IE8.

[Tade0]

I had to correct the condition under which one test is ignored, but overall I'm seeing that this fix brings an improvement.

Fixed with ​git:d7e067154ed0407a355c77266e6f2f93ca81bf93.